The present invention generally relates to nut plate fastener assemblies, and more specifically relates to a rivetless nut plate fastener assembly which is configured for use with a composite material workpiece.
Fasteners are used in the aerospace industry for securing at least two workpieces together. Fasteners used in such installations may include a nut plate and a nut which are part of an assembly. There are many different design configurations of nut plates being used today. Two major classes are riveted nut plates and rivetless nut plates.
In riveted nut plates, two rivets are employed for attaching the body of the nut plate to the workpiece. To eliminate the potential for leakage through the openings of the riveted nut plates, sealant is used between the workpiece and the nut plate.
With regard to rivetless nut plates, some designs provide that a sleeve is flared against a workpiece. One example of this type of rivetless nut plate is disclosed in U.S. Pat. No. 4,732,518, which is hereby incorporated herein by reference in its entirety. The '518 patent illustrates the insertion of a sleeve inside a workpiece against heavy interference forces and then deformation of the sleeve to produce flaring of the end of the sleeve. The sleeve has a serration/lobe configuration thereon with the serration/lobe configuration being long and tapered such that the serrations/lobes extend into the walls of the workpiece. The tapered feature, length and specific geometry are necessary to make installation possible with the method of installation which was chosen for its application. The main object of the '518 patent with its tapered and extended serration/lobe configuration was to enhance the fatigue life of the workpiece by distributing the load throughout the workpiece and providing expansion due to the insertion of the sleeve into the workpiece, and to cold work the material adjacent the perimeter of the workpiece aperture.
Other nut plate designs do not rely on flaring of the sleeve. U.S. Pat. Nos. 5,096,349, 5,245,743, 5,405,228; 7,059,816 and pending U.S. patent application Ser. No. 10/929,701 (filed Aug. 30, 2004) disclose rivetless nut plate designs which do not rely on flaring of the sleeve, and these five items are hereby incorporated herein by reference in their entirety. While some rivetless nut plate designs rely on adhesive for attaching the nut plate to the structure, the designs disclosed in the five items cited above rely on heavily cold-worked holes and high interference engagement utilizing a hardened pin as the installation tool to expand a sleeve element into engagement with a workpiece structure. Because of high level expansion, the friction forces created are intended to retain the nut plate and provide expected mechanical properties.
Currently within the aerospace industry, the rivetless nut plate disclosed in U.S. Pat. No. 7,059,816 is becoming more and more predominate within aluminum structure applications, taking the place of outdated nut plates utilizing satellite rivets. This is due to the time saving nature of the rivetless nut plate, while still maintaining the mechanical properties for torque out and push out as required by NASM25027.
This rivetless nut plate works by pulling a mandrel through the inside diameter of a nut plate retainer which has been placed in a pre-drilled hole, expanding the fastener sleeve radially in the hole. This radial expansion of the fastener sleeve in the work piece embeds the sleeve with its complex lobe design into the pre-drilled hole and creates the interlocking action between the nut plate and the structure required to resist torque out and push out.
Unfortunately, with composite structures, the introduction of holes in the material and subsequently fasteners in those holes, create unique problems not existing with most aerospace structural materials. Due to the properties of composites, too much radial expansion of the work hole caused by the fastener sleeve leads to delamination of the composite, while too little radial expansion hampers proper shear load transfer. A solution is needed. Thus, the present invention deals with rivetless nut plates for installation in composite structures.
While it seems that there is no current approach to modifying a rivetless nut plate such that it is useable with composite material workpieces, there is an approach using the outdated nut plate with satellite rivets. This approach requires drilling and reaming operations for their holes, or it needs to be accompanied with a bonding agent. Due to the nature of composite material, each drilled hole, as well as each additional installed fastener, endangers the integrity of the structure. Performing the drilling process in the composite material is difficult and costly, and can also damage the structure by exposing fibers to water absorption problems, while each installed fastener can damage the composite structure due to overfill conditions. In overfill conditions, the excessive radial expansion of the fastener can lead to delamination of the composite structure. Finally, the integrity of the bonding process is not easily verifiable and it can deteriorate due to environmental changes such as heat, moisture and chemicals.
Thus, an embodiment of the present invention aims to fasten a rivetless nut plate to a composite structure without the use of bonding agents, additional holes or satellite rivets, and without damaging the structure.